The amount of digital data, particularly file data, has been increasing rapidly. An NAS (Network Attached Storage) is a storage device suitable for allowing a large number of computers to share file data via a network. At present, many file data storages utilize NAS devices.
Digital data including data files needs to be stored for a long period for a variety of purposes, for example, in order to meet various legal requirements. A CAS (Content Addressed Storage) ensures invariance of data to provide a solution for long-term data archiving. In general, the current data is stored in an NAS device as long as the data is used, and is subsequently migrated to a CAS device for archiving. The migrated data is also called archive data. For example, E-mail data on an NAS device may be archived in a CAS device in order to comply with the relevant regulation.
When a data file is archived, the path name of the archived file is changed. For example, the path name of a file A is changed from //NAS-A/share/fileA to //CAS-A/archive/fileA. To allow a NAS client to be notified of the change in the file path name, stub information (also called a stub or stub data) is generated in the NAS device. The stub includes a source location in the NAS device and a file destination location in the CAS device. The stub information allows the NAS client to know that the file has been migrated and that the path name has been changed as a result of the archiving. The stub information includes the destination file path of the migrated file. The NAS client can use the stub information to determine where the archived file data is actually present. Furthermore, the NAS device and the CAS device can use a GNS (Global Namespace) to integrate namespaces.
The NAS device needs to be backed up in order to prevent data loss when the NAS device is faulty. If the NAS device and the CAS device use the GNS to integrate namespaces, backing up even the actual archive data in the CAS device wastes backup time and the storage capacity of the backup destination. Thus, as disclosed in, for example, Patent Literature 1, the device determines whether an access from the NAS client is a normal NAS access or a special NAS access for backup. If the access is for backup, the actual archive data in the CAS device is not backed up but only the stub information is backed up.